The performance of disk storage systems is limited by the speed of mechanical components that are used for positioning and transferring data. Since RAID storage systems utilize multiple drives, it is possible to improve their performance by using the hardware of all drives in parallel. Drive spanning allows multiple physical disk drives to be logically concatenated into a single logical disk drive. The capacity of the logical drive created via spanning is the capacity of the physical drives times the number of physical drives. A technique called data striping distributes data evenly across the physical drives in such a manner as to maximize input/output (I/O) performance. Striping divides the logical drive into data blocks called stripes, which are then distributed over the physical disk drives. The layout is such that a sequential read of data on the logical drive results in parallel reads to each of the physical drives. This results in improved performance since multiple drives are operating simultaneously.
Striping is used in the implementation of most of the basic, single RAID levels and as a consequence, any multiple RAID levels that employ the single RAID levels.
The simplest RAID level, RAID 0, uses block-level striping without parity. In RAID 0, one file is divided into multiple groups of sectors and striped across multiple disks. Two advantages of RAID 0 include a large logical disk drive through drive spanning and performance acceleration through data striping. The major uses of RAID 0 are in situations where no redundancy is required or where redundancy can be provided through the use of transaction logs that recreate data from the last status recorded in the log. As a result, RAID 0 is often employed by users seeking high performance at low cost. However, the disadvantage of RAID 0 is that it provides no redundancy. If one disk fails, all data is lost and all disks must be reformatted. RAID 0 does not support a hot-spare drive, thus, if one drive fails, all data is lost. For comparison purposes, RAID 0 is regarded as the baseline against which to measure the performance of the other RAID levels.
Another RAID level, RAID enhanced (also known as RAID 1E, Hybrid RAID 1, or RAID 6), combines mirroring with data striping—data is striped across each disk in the array. The first set of stripes includes the data stripes, and the second set of stripes is the mirror (copies) of the first data stripe, but shifted one drive. RAID 1E shares the characteristics of RAID 1 but additionally allows more than two drives, including odd numbers of drives.
When the host operating system sends an I/O request to move data between the disk and its memory, it provides a scatter gather list (SGL) that describes the actual memory regions to move the data to or from. Typically, the memory regions are fragmented and data is scattered to the various regions when data is transferred to host memory space. When memory is retrieved from host memory space and written to a drive, it is gathered from the various memory regions to form scatter gather list. Typically, striping is implemented by taking the Scatter Gather List (SGL) for the transfer data, which provides a description of the location of the data, and formulating a new SGL for the data transfer to each drive. The original operating system host I/O request is divided into several new I/O requests to be sent to each of the stripe disk members and break up the original scatter gather list is divided into several smaller lists (one for each new I/O request). This method requires extra processing and memory resources to parse the original SGL and create new smaller ones dedicated for each of the new I/O requests.
Therefore, it would be desirable to provide a code efficient method of fulfilling a host I/O request that does not require extra processing and extra memory resources.